The present invention relates to a method of molding elastomeric article.
When a pneumatic tire which is a typical elastomeric article is molded or vulcanized, conventionally, a green tire is first built and in a mold it is heated by steam under a constant pressure P1U for a predetermined time T1 and then the inside thereof is pressurized by a gas at a constant high pressure P2U for a predetermined T2 as shown in FIG. 5. By pressurizing the inside, the softened rubber is pressed against the inner surface of the mold.
However, when the inner surface of the mold is provided with relatively deep hollows for example as a negative tire tread pattern, it is difficult to fill all the corners of the hollows with rubber, and defective molding such as rubber bareness on the outer surface of the tire is liable to occur.
In general, in order to prevent rubber bareness, a large number of vent holes are provided in the hollows. Accordingly, a large number of spews of rubber are formed on the molded article. It takes a lot of time and labor to remove the spews. The time and labor may be reduced if the number of the vent holes is decreased, but rubber bareness increases.
It is therefore, an object of the present invention to provide a method of molding an elastomeric article, in which the pushing of the elastomer into hollows on the inner surface of the mold is improved to prevent the occurrence of bareness of elastomer on the surface of the molded elastomeric article and also to decrease the number of bent holes.
According to the present invention, a method of molding an elastomeric article comprises
putting an elastomeric article in a mold,
softening the elastomeric article in the mold by heating the elastomeric article,
pressing the elastomeric article against the mold by pressurizing an inside of the elastomeric article by letting a fluid therein, and
changing the pressure of said fluid in a short cycle so as to beat the elastomeric article against the mold repeatedly.
The cyclic pressure change must be carried out after the elastomeric article is softened at latest. Each cycle of the pressure change is made up of a decompression step in which the pressure decreases from a higher pressure to a lower pressure, and a subsequent repressurizing step in which the pressure increases from the lower pressure to the higher pressure. The number of cycles, namely the number of the repressurizing or decompression steps is at least two but at most about 50, usually at most about 30, preferably at most 20. The duration time of one decompression step is not more than about 60 seconds, and the duration time of one repressurizing step is also not more than about 60 seconds. If these duration times are longer than 60 seconds, the beating effect decreases, and it is difficult to prevent the bareness of elastomer.
Taking a method of vulcanizing a pneumatic tire for example, embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings.